The present invention relates to equipment and a manufacturing method for manufacturing a soot preform for an optical fiber, photomask or the like.
For manufacturing a soot preform there are widely used methods, such as the VAD method and the OVD method. According to these methods, the gas of raw materials such as silicon tetrachloride and germanium tetrachloride is supplied to a burner together with a combustion gas and the like, and glass particulates that are formed by hydrolysis reaction therefrom are deposited on the tip of or around a starting rod. The soot preform thus produced is dehydrated and sintered, resulting in a transparent vitrified preform. The transparent glass preform is melted by heating, and drawn to form an optical fiber.
FIG. 4 is a sectional view illustrating the main part of equipment in which a soot preform is produced. The numeral 1 shows a reaction vessel, 2 a burner, 2a flame, 3 an air inlet, 4 an exhaust port, 5 a starting rod, 6 a soot preform, and 7 the flow of an air current.
Gases, such as a raw material gas comprising silicon tetrachloride, germanium tetrachloride, etc., a combustion gas such as hydrogen, oxygen, etc., and a separating gas such as argon which delays the mixing of hydrogen and oxygen, are supplied to the burner 2. Then, glass particulates made of quartz simple or quartz substance in which germanium oxidize, etc. is doped are produced in the flame 2a by hydrolysis reaction. Then, the glass particulates thus produced are deposited on the tip of the starting rod 5 or around the starting rod. The starting rod 5 is drawn up while turning around its axis. The deposit of the glass particulates grows in the radial and longitudinal directions of the starting rod, forming the soot preform 6 having an approximately columnar shape.
Part of the glass particulates generated in the flame 2a of the burner 2 are, without accumulating on the soot preform 6, allowed to flow upward by the flow of the high-temperature gas and float on the air current 7 in the reaction vessel. Then, the glass particulates thus floating, after the temperature declines by some degrees, adhere to the surface of the already formed soot preform, as well as the inner wall surface of the reaction vessel, in a different condition as compared with the bulk density of the preform.
When the glass particulates having adhered to the wall surface of the reaction vessel grow and fall off by of their own weight, etc, they will float in the reaction vessel and also adhere to the surface of the soot preform as described above.
The glass particulates having floated as described above and not deposited directly on the preform are in a state where the temperature has become lower as compared with the glass particulates that have been directly deposited on the preform. Accordingly, they differ in the physical properties such as the bulk density, and hence cause voids in the transparent glass preform. The voids tend to break an optical fiber when it is drawn from the transparent glass preform, or deteriorate the optical transmission characteristics of the optical fiber.
The equipment and method for manufacturing a soot preform according to the present invention can restrain the floating of glass particulates in the reaction vessel that would occur if a soot preform is manufactured according to the conventional techniques. The novel equipment and method enables the manufacture of a soot preform from which a transparent glass preform of good quality, and hence a good quality optical fiber, can be produced.
The equipment for manufacturing a soot preform according to the present invention is, as in the case of conventional equipment, provided with a reaction vessel, a burner that generates glass particulates, and a starting rod on which the glass particulates are deposited.
The glass particulates are generated by hydrolysis reaction in the flame formed by burning the combustion gas that is supplied together with raw material gas to the burner provided in the reaction vessel. As in the case of conventional equipment, a soot preform is manufactured in an approximately columnar shape by depositing glass particulates to grow on the tip of the starting rod or around the starting rod while turning the starting rod around its axis and drawing it upward. The equipment according to the present invention is characterized in that a partition board is provided in part of the space around the soot preform in the reaction vessel such that the space is separated into upper and lower parts, and an exhaust port is provided below the partition board in the side wall of the reaction vessel and the burner is positioned in the space below the partition board. By such arrangement, the floating of the glass particulates in the reaction vessel can be limited to the lower part and hence the adhesion of the floating glass particulates to the soot preform can be reduced. As a result, the number of voids formed in the preform can also be reduced when the soot preform is vitrified to transparent glass.